Production of components of electronic devices such as transistors, solar cells, light emitting diodes, and similar devices from inorganic semiconductors based on silicon, germanium, or gallium arsenide is very well known in the art, as is the use of dopants in the manufacture of such inorganic semiconductors. Atoms of “n-dopant” inorganic elements that are electrically neutral, but comprise one or more extra valence electrons as compared to the basic inorganic semiconductor material are typically doped directly into the inorganic semiconductor lattice as impurities. and thereby provide potentially current-carrying electrons into the delocalized conduction bands that occur in such “n-type” inorganic semiconductors. Technology for directly “doping” traditional inorganic semiconductors produces electronic semiconductors with very good electrical performance, but the production costs can be very high, and the rigid and expensive inorganic semiconductors are not suitable for some very desirable end use applications of high potential interest.
There has been much recent work directed toward developing large area and/or “printable” electronic components and devices based on organic semiconductors (organic small molecules, oligomers, or polymers) that can potentially be solution processed at much lower cost, perhaps on flexible substrates such as plastic or paper, so that many new potential circuits, electronic devices, or end-use applications can be developed.
There are important differences between the inorganic and organic semiconductors. For example, there are generally not completely delocalized “bands” or “conduction states” for electrons or holes in organic semiconductors that extend throughout the bulk solids. While hole and/or electrons can migrate within the conjugated π orbitals of unsaturated organic semiconductor molecules, macroscopic conduction of current is typically limited by quantum mechanical “hopping” of holes or electrons between neighboring but distinct organic molecules in the solid state.
Controlled chemical doping of organic semiconductor materials is known in the art as a technique to improve the electrical conductivity and/or electrical performance of some types of organic semiconducting materials and/or devices. See for example Walzer et al, Chem. Rev. 2007, 107, 1233-127, which describes both general principals of doping in organic semiconductors, and many specific applications in organic light emitting diodes (“OLEDs”), and organic photovoltaic cells (“OPV”). In n-type doping of organic semiconductors, the n-type dopants typically reductively donate electrons into the lowest unoccupied molecular orbitals (“LUMO”) of the organic semiconductor compounds, to form at least a few anions among the remaining undoped organic semiconductor molecules. and the n-dopant compound is oxidized to a cation in the process. In the presence of an electrical potential difference, the electrons injected into the LUMOs typically “hop” between the n-doped organic semiconductor molecules, to carry the electrical current.
It is believed in the art that two major functions of directly n-doping organic semiconductor compositions, are (i) increasing the density of “free” electron current carriers available for conduction, and/or (ii) preferential filling of deep trap states, a trap typically being associated with an impurity or a defect with a lower lying empty orbital than the semiconductor, or a flaw, i.e. static or dynamic disorder, in the solid state structure, thereby reducing the activation energy required for the “hopping” transport process of the electrons from organic molecule to organic molecule. Additionally, directly doping organic materials at electrode interfaces can reduce contact resistances by providing improved electron or hole tunneling through narrow interface depletion regions, and by manipulating the molecular energy level alignments at organic-organic hetero-junctions; these reductions can sometimes provide orders-of-magnitude increases in the conductivity of organic films. The positively charged cation derived from the n-dopant material may position itself within the semiconductor solid, and/or disrupt its solid state structure in many ways. If the cations derived from an n-dopant molecule are small (such as alkali metal cations), they will be less likely to disrupt the semiconductor's solid structure; however, they may undesirably migrate within the organic lattice and/or between different structures within a doped device, as well as acting as electrostatic traps for electrons on neighboring semiconductor molecules.
Moreover, as noted by Walzer et al, and many others of skill in the art, “In contrast to p-type doping, n-type molecular doping is intrinsically more difficult . . . . For efficient doping, the HOMO level of the dopant must be energetically above the LUMO level of the matrix material which makes such materials unstable against oxygen. With increasing LUMO energy, the difficulty to find suitable materials is increased.” It has indeed proven difficult to identify stable and easily processable n-type dopant molecules that are sufficiently strong reducing agents as to be able to dope electrons into the high lying LUMOs of many organic semiconductors of interest. When sufficiently strong n-dopants have been identified, they have themselves often been chemically unstable, or unstable if exposed to air or water while being processed, or after processing. In many cases the n-doped semiconductors themselves are also unstable to oxidation by air and/or water.
Accordingly, there remains an unmet need in the art for much improved n-dopant materials, and methods for their use, and improved n-doped semiconductor compositions, in order to make improved and economically viable n-doped organic semiconductors devices comprising those n-doped organic semiconductors, for applications in organic field effect transistors, organic solar cells, and organic light emitting diodes. It is to that end that the various embodiments of the inventions described below are directed.
WO 2005/036667 and its US Equivalent US 2007/0278479 disclosed the use of certain highly reducing monomeric and electrochemically generated ruthenium terpyridine and chromium bipyridine complexes as reducing agents for fullerenes, and zinc phthalocyanine.
The cobaltocenium cation (structure shown below) has long been known in the chemical prior art as the cation of stable “18 electron” diamagnetic salts. It has also been known in the art that the cobaltocenium cation can be reduced by one electron, by very strong inorganic reducing agents such as alkali metals, to form a neutral and monomeric “19 electron” monomeric radical compound “cobaltocene” (“Co(C5H5)2”). Decamethylcobaltocene (“Co(C5Me5)2”) and the corresponding decamethylcobaltocenium cation are also known in the chemical arts.

While such electrically neutral 19 electron cobaltocene compounds can be isolated, they are extremely reactive reductants towards potential oxidants such as oxygen and water, and therefore are difficult to make, handle, store, and use under ambient conditions, or in most industrial processes.
Domrachev et al (Russ. Chem. Bull. 43(8) 1305-1309, 1994) reported the use of neutral “19 electron” cobaltocene to reduce and/or n-dope C60 and/or C70 films, to induce the formation of at least some cobaltocenium fulleride salts in the doped fullerene films.
In recent years, cobaltocene and decamethylcobaltocene were also described as n-dopants for films of organic semiconductor compounds, such as copper phthalocyanine (CuPc) and pentacene. See for example Chan et al, Chem. Phys. Lett. 43 1 (2006) 67-71; Chan et al, Organic Electronics 9(2008) 575-581, Chan et al, Appl. Phys. Lett 94, 2003306 (2009), Chan and Kahn, Appl. Phys. A (2009) 95 7-13.
U.S. Patent Publication 2007/029594 disclosed and claimed the use of a variety of organometallic compounds, having a wide range of reducing power, including numerous monomeric metallocenes and other sandwich compounds, as potential n-dopants for organic semiconductors. However, many of the monomeric sandwich compounds disclosed therein are not “19 electron” radical compounds, and are not sufficiently strong reducing agents to effectively n-dope many organic semiconductor compounds or compositions of interest which have relatively low electron affinities. That patent publication more specifically described a few monomeric 19 electron sandwich compounds, such as cobaltocene, decamethyl cobaltocene, and two substituted monomeric iron-based sandwich compounds (Fe(C5H5)(C6Me6), and (Fe(C5Me5)(C6Me6)). US 2007/029594 suggested that those monomeric 19 electron compounds were effective n-dopants, and that such 19 electron metallosandwich n-dopants were superior in some respects to previously known inorganic dopants such as lithium metal, because the larger metallosandwich cations produced by the disclosed doping processes were more resistant to undesirable mobility of the cations of the dopants within the semiconductor solids. US 2007/029594 did not however disclose or suggest methods of making and using dimeric or oligomeric metallocene compounds in its processes, or that dimeric or oligomeric bis-metallosandwich compounds would be effective n-dopants, or any other advantage of employing bis-metallosandwich compounds as n-dopants.
The literature does disclose a few attempts to reduce certain 18 electron metallosandwich cations to produce highly reducing 19 electron monomeric metallosandwich radicals, but with certain other 1 8 electron metallosandwich cations those attempts often result in dimerization of the 19 electron monomer, with the formation of a carbon-carbon bond between two 18 electron monomer fragments. For example, Gusev et al (J. Orgmet. Chem, 452, 219-222, 1993), which disclosed that an attempt to electrochemically reduce salts of the 18 electron rhodocenium cation, rather than producing a stable and neutral 19 electron rhodocene monomer, actually induced formation of a carbon-carbon bond between the rings of the rhodocene monomers, to form a reasonably chemically and air stable and isolated “rhodocene dimer,” as shown below. Similar dimeric iron and iridium based metallocenes derived by reduction of [Fe(η5-C5H5)(η6-C6MenH6-n)]+ (n=0-5), [Fe(η5-C5Me5)(η6-C6H6)]+, [Ru(η5-C5H5)η6C6H6)]+ (R═H, Me), and [Ir(η5-C5R′5)(η5-C5R″5)]+ (R′, R″═H, Me) salts have also been subsequently reported by Gusev et al and others.
However, neither Gusev et al, nor prior art reports (see Gusev, et al, J. Organomet. Chem. 1997, 531, 95-100, Hamon, et al, Am. Chem. Soc. 1981, 103, 758-766, Murr et al, Inorg. Chem. 1979, 1 8, 1443-1446, and Fischer, et al, Organomet. Chem. 1966, 5, 559-567) suggested that such dimeric metallocene compounds, which comprise two 18 electron sandwich compounds linked by a carbon-carbon bond are useful as an n-dopant for organic semiconductors.
Applicants have discovered that some bis-metallosandwich compounds described in the prior art and other bis-metallosandwich compounds described below can unexpectedly serve as unexpectedly strong, yet reasonably air and/or water stable reducing agents and/or n-dopants for a variety of organic semiconductors. Moreover, use of Applicants' methods and/or the bis-metallosandwich n-dopants described below allow the use of solution processing to produce new and unexpectedly useful n-doped compositions and organic electronic devices comprising such n-doped compositions.